The physical basis of the Lake Mikri Prespa systems: geology,climate, hydrology and water quality

Lake Mikri Prespa is 47 km² in area, with amaximum depth of 8.4 m and a normal water level ataround 850 m above sea level. It collects water froma granitic and karstic catchment of 189 km² whichincludes snowy mountains rising to over 2000 m. Themain flat land is the sedimentary plain to the eastand the sandy isthmus that separates Mikri Prespa fromthe much larger Megali Prespa into which the formernormally drains. Rainfall averages around 750 mmbecause the 600 mm to 700 mm falling on the plains isaugmented by heavier falls including much snow on themountains. There is rainfall each month with anautumnal maximum. River inputs to the lake and thelake level itself peak in the spring with thesnowmelt. Mikri Prespa normally rises by about a metreto flood the surrounding wet meadows landward of thereed fringe. The annual cycle of water levelfluctuation is superimposed on infrequent upwardsurges in the level of Mikri and Megali Prespa becauseof particularly wet and snowy winters and, in recentyears, the steady decline of the level of MegaliPrespa because of tectonic activity.There has been considerable human modification of thehydrology of the area with the diversion of the AgiosGermanos torrent from Mikri to Megali Prespa, thecreation of a canal in Albania which can input orwithdraw water from the lake, the culverting of thecanal linking Mikri and Megali Prespa, and thecreation of an irrigation scheme taking water directlyfrom Mikri Prespa and from the Agios Germanosstream.The lake water is base rich because of the limestonein the catchment. Whilst concern has been expressed atthe eutrophication of the lake, recent studies haveproved that there has been no significant change innutrient status this century and the lake is noteutrophic. There has been a significant increase inturbidity but this may be the result of sedimentdisturbance by an introduced fish species.The lake and its supporting hydrological system willneed careful monitoring if it is to be effectivelymanaged. A particularly high priority is thedevelopment and implementation of a water levelmanagement plan for the lake.     

Mollusc biodiversity in a European ancient lake system: lakes Prespa and Mikri Prespa in the Balkans

The spatial distribution of (endemic) biodiversity in ancient and potentially ancient lakes in Europe is poorly understood. Examples include Lakes Prespa and Mikri Prespa in the Central Balkans. Utilizing information of the most species-rich taxon in these lakes, the Mollusca, we therefore attempt to statistically assess and visualize the spatial distribution of biodiversity, to analyse biogeographical patterns, and to carry out a conservation assessment. We estimate that at least 40 (sub)species (29 gastropod and 11 bivalve taxa) occur in the lakes. For both lakes combined, 37.5% of the mollusc taxa are endemic. In general, the mollusc richness in Lake Mikri Prespa is lower than in Lake Prespa and less heterogeneously distributed. The highest species richness can be seen on the western and south eastern shores of Lake Prespa. Based on the presence/absence of genera, a minimum spanning tree analysis supports the sister lake relationship of both lakes, which, in turn, are most closely related to lakes in the western Balkans and not to nearby Lake Ohrid. The IUCN red list assessment revealed (A) a tendency towards mollusc faunal change, (B) a contemporary decline and potential loss of mollusc diversity, and (C) that all endemic species are of conservation concern.     

The importance of Prespa National Park for breeding and wintering birds

The breeding avifauna of Prespa National Park is ofnational and international importance both due to itshigh richness and the internationally importantpopulations of rare species. The latter include theDalmatian (Pelecanus crispus) and Great White(Pelecanus onocrotalus) pelicans and the PygmyCormorant (Phalacrocorax pygmaeus). Populationsof national interest include Cormorant (Phalacrocorax carbo), Greylag Goose (Anseranser), Goosander (Mergus merganser), GreatWhite Egret (Egretta alba) and six otherspecies of ardeids. Among the 261 bird speciesobserved in the area of ca 250 km² since thelate 60s, 164 breed and 41 are very rarely observed.Passage migrants and winter visitors include 91species. Twelve breeding species became extinct in theNational Park during the last 25 years and others areendangered. Raptors and aquatic birds suffered moredue to habitat and land use alterations brought aboutby the construction of an irrigation scheme.Eighty-one species observed in the area are listed asin need of special protection in Annex I of the 79/409EEC Directive and 32 of them breed. Lake Mikri Prespais of virtually no importance for wintering waterfowlsince it is frozen every winter, but the Greek part ofLake Megali Prespa, which never freezes, concentratesamong other species the 20% and 35% respectively ofthe Great Crested Grebes and Tufted Ducks wintering inGreece.     

Co-management and Spatial Features Contribute to Secure Fish Abundance and Fishing Yields in Tropical Floodplain Lakes

Empirical data are needed to show the efficacy of co-management, which is regarded as a promising approach to achieve conservation goals. In this study, we addressed the potential influence of fisheries co-management to increase fish abundance and fishing yields in the lower Tocantins River Basin (Brazilian Amazon), downstream from a large dam. We analyzed 590 fish landings (6.7 t of fish) from five fishing villages and 48 fish samples obtained using gillnets (10,378 fish from 101 species) in 12 floodplain lakes in four regions: two with incipient co-management and two unmanaged. The fish species richness did not differ among the regions, but the lakes in the regions that were co-managed had higher fish abundance (biomass and number of individuals) and a higher mean proportion of fish reproducing during the high water season. Fishers had higher catches per unit of effort in the co-managed regions than fishers in the non-managed regions. These results were also influenced by geographic factors (distance and accessibility of lakes), as fish biomass was higher in lakes that were distant from the main river and from the main city in the region. Managers should thus consider strategic selection of the geographic locations of managed sites, even in remote areas. However, the fish biomass sampled in lakes was more related to region than to the lakes’ geographical location. Therefore, co-management has at least partially contributed to increased fish abundance and fishing yields in the studied region, through the protection of an important fish habitat (lakes). We provide empirical evidence that co-management can contribute to the maintenance of fish abundance, sustainability of fisheries, and food security in large tropical rivers impacted by damming.     

African lakes and their fishes: conservation scenarios and suggestions

Synopsis Man has been associated with a variety of lakes throughout his evolutionary history in Africa. Lakeside dwellers have a close association with and understanding of these lakes. In the past four decades, however, overexploitation, introductions of alien fishes and the possibility of oil pollution present frightening prospects of irreversible losses and massive extinction. The development of scientific understanding of the life support processes, the ecosystems and the rich communities of these lakes has been so outpaced by exploitation and manipulations that accurate predictions regarding the future are impossible. Shallow lakes are more sensitive to physico-chemical changes caused by climatic as well as agricultural and industrial development than deep lakes, but their biotae are endowed with a resilience which facilitates recovery from major depletions to population size. In contrast, the speciose endemic cichlid faunae of deep lakes are sensitive to fishing pressure, are awkward to manage and should be regarded as representing a much smaller resource than initially imagined. The clupeids of Lake Tanganyika can sustain intensive fishing, but their introduction into other natural lakes is not recommended. Enormous changes to native faunae followed the introduction of Lates niloticus to lakes Kyoga and Victoria with dramatic consequences for the fisheries, for the socio-economic status of the region and for the maintenance of biotic diversity. Extinction of almost 300 fish species is a possible further consequence of L. niloticus predation. Intensive selective fishing for L. niloticus is being initiated, but is unlikely to resolve the problem. In the time necessary to substantially reduce L. niloticus, numerous endemic fishes may suffer extinction. A viable alternative to extinction of these species is captive propagation. Conservation of fishes by captive propagation is not a common or well documented practice. To encourage the acceptance of this conservation option a theoretical scenario in which the concepts and protocols are applied to the fishes of Lake Victoria is given. The possibility of returning rescued populations to the lake at a later date, assuming L. niloticus populations have been reduced, is also discussed. It is recommended that captive propagation should be practised to conserve species and to retain the option of returning rescued taxa. Scientists are urged to seek the funding to study tropical ecosystems so that conservation and rational development may acquire a sound foundation.     

Landscape variables affecting fishery yield in lake systems of the Central Amazon region, Brazil

According to fisheries data, lakes are important systems for fish production in the Amazon basin. However, there is no information about the relationship between landscape variables and fishing yield that allows foresight into potential resource exploitation in this environment. The present study aims to evaluate this relationship with the hypothesis: lakes of different shapes give the same fishery yield in the Amazon, after considering the effects of lake size, distance to the river, fishing effort, fuel and ice used. Fishery data from 1994 to 1996 were analyzed with regard to 3228 trips on 50 lakes of the main white water tributaries of the Amazon basin. Analysis of covariance was applied to test this hypothesis. With variables such as fishing grounds access, fishing effort and lake shape the model explained a significant 72% of variabilities in the fisheries yield. Fishing yields among lake systems were different, thus the null hypothesis was rejected (P < 0.05). Results indicate that dendritic lakes far distant from the main river have greater productivity than floodplain lakes because there are more habitats of fish refuge for reproduction and feed available to the fish; there are also more limitations to access by predators.     

Invaders or endemics? Molecular phylogenetics,biogeography and systematics of Dreissena in the Balkans

1. Zebra mussels and their relatives (Dreissena spp.) have been well studied in eastern, central and western Europe as well as in North America, because of their invasiveness and economic importance. Much less is known about the biology and biogeography of indigenous (endemic) taxa of Dreissena, in the Balkans. A better knowledge of these taxa could help us (i) understand the factors triggering invasiveness in some taxa and (ii) identify other potentially invasive species. 2. Using a phylogenetic approach (2108 base pairs from three gene fragments), Dreissena spp. from natural lakes in the Balkans were studied to test whether invasive Dreissena populations occur in such lakes on the Balkan Peninsula, whether Dreissena stankovici really is endemic to the ancient Lakes Ohrid and Prespa, and to infer the phylogenetic and biogeographical relationships of Balkan dreissenids. 3. No invasive species of Dreissena, such as Dreissena polymorpha, were recorded. The supposedly ‘endemic’D. stankovici is not restricted to the ancient Lakes Ohrid and Prespa, but is the most widespread and dominant species in the west‐central Balkans. Its southern sister taxon, Dreissena blanci, occurs sympatrically with D. stankovici in Lakes Prespa, Mikri Prespa and Pamvotis. Both species are classified into the subgenus Dreissena (Carinodreissena) of which the subgenus Dreissena (Dreissena) (which includes the invasive D. polymorpha) is the sister taxon. Dreissena blanci and D. stankovici are considered to represent distinct species. 4. On a global scale, the two Balkan species have small ranges. An early Pliocene time frame for the divergence of the subgenera Carinodreissena and Dreissena is discussed, as well as potential colonization routes of the most recent common ancestor of Carinodreissena spp. 5. The ambiguous taxonomy of dreissenids in the Balkans is addressed. As nominal D. blanci presbensis from Lake Prespa has nomenclatural priority over D. stankovici, the correct name for the latter taxon should be Dreissena presbensis.     

Enhancement and conservation of inland fisheries resources in China

The inland aquatic ecosystem in China have been largely influenced by large-scale economic activities and over-exploitation of aquatic resources. Fisheries resource enhancement and conservation activities have been carried out throughout China. These activities include establishing artificial fish habitat and hatchery, releasing juvenile fish to rivers and lakes, forbidding illegal fishing, and setting up natural reserves. The purpose of these activities is to restore the quantity of inland fisheries resources, guarantee sustainable development of fisheries, maintain biological diversity and keep an ecological balance. These activities, conducted with a variety of methods, numerous species, extensive water areas and large scale programs have played an active role in fisheries resource enhancement and conservation. As a consequence the inland water environment has been improved; the biological diversity and fish catch increased, and the social, ecological and economic benefits have been significantly improved. However, problems such as inappropriate technology and nonstandard operations continue to affect fisheries resource enhancement, conservation and ecological stability. This paper reviews the history and practice of inland fisheries resource enhancement and conservation in China, analyzes the existing problems and deficiencies in the activities, and finally make some recommendations on technology and procedures to sustain inland fisheries resources.     

Marine fisheries as ecological experiments

There are many examples of ecological theory informing fishery management. Yet fisheries also provide tremendous opportunities to test ecological theory through large-scale, repeated, and well-documented perturbations of natural systems. Although treating fisheries as experiments presents several challenges, few comparable tests exist at the ecosystem scale. Experimental manipulations of fish populations in lakes have been widely used to develop and test ecological theory. Controlled manipulation of fish populations in open marine systems is rarely possible, but fisheries data provide a valuable substitute for such manipulations. To highlight the value of marine fisheries data, we review leading ecological theories that have been empirically tested using such data. For example, density dependence has been examined through meta-analysis of spawning stock and recruitment data to show that compensation (higher population growth) occurs commonly when populations are reduced to low levels, while depensation (the Allee effect) is rare. As populations decline, spatial changes typically involve populations contracting into high-density core habitats while abandoning less productive habitats. Fishing down predators may result in trophic cascades, possibly shifting entire ecosystems into alternate stable states, although alternate states can be maintained by both ecological processes and continued fishing pressure. Conversely, depleting low trophic level groups may affect central-place foragers, although these bottom–up effects rarely appear to impact fish—perhaps because many fish populations have been reduced to the point that they are no longer prey limited. Fisheries provide empirical tests for diversity–stability relations: catch data suggest that more diverse systems recover faster and provide more stable returns than less diverse systems. Fisheries have also provided examples of the tragedy of the commons, as well as counter-examples where common property resources have been managed successfully. We also address two barriers to use of fisheries data to answer ecological questions: differences in terminology for similar concepts and misuse of records of fishery landings (catch data) as a proxy for biomass trends.     

Unique qualities and special problems of the African Great Lakes

Synopsis The African Great Lakes consist of large, deep rift valley lakes (e.g. Malawi & Tanganyika) and shallower lakes between the Eastern and Western Rifts (e.g. Victoria). They are a group comparable in size to the North American Great Lakes, but are old. Most are seasonally thermally stratified, and wind is the decisive factor that determines the annual cycle of cooling and mixing. Lakes Tanganyika, Malawi and Kivu are meromictic, with deep relict hypolimnia. Large magnitudes and time scales of periodic internal motion, where these have been measured, appear unique among lakes. These lakes harbour the world's richest lacustrine fish faunas, and the family Cichlidae provides the supreme example of geographically circumscribed vertebrate evolution. The lakes provide a unique comparative series of natural laboratories for evolutionary studies. Primary production is generally high, but in the deeper lakes standing stocks of plankton and of small fish species are low. These pelagic populations are characterised by very high P:B ratios. The fisheries are productive and of socio-economic importance. Large-scale mechanised fishing is not compatible with the survival of the diverse fish communities. Cichlids appear especially vulnerable to unselective fishing. Aquatic reserves might offer a means of survival for at least some communities. Various pollution threats exist. Because water retention times are long, extremely long for some deep lakes, and flushing rates are low, the lakes are vulnerable to pollution which would be long-lasting. Introductions of alien fishes have mostly had undesirable or disastrous results. While the faunas are one of the significant natural heritages of mankind, their conservation must realistically be linked to the legitimate development of the lakes for the well-being of the people who live there. Scientific value alone will not protect the lakes. Just as survival of African terrestrial wildlife in extensive reserves depends heavily upon tourism, so also might the cichlid flocks in underwater reserves. Greater interest from the international scientific community is needed to further rational development and conservation of these great lakes.Invited Editorial     

Lake Prespa,a European natural monument,endangered by irrigation and eutrophication?

The most interesting (Hutchinson, 1957), and at the same time oldest graben lakes of Europe, are Lakes Ohrid and Prespa. According to geologists and geographers, both originated during the Pliocene. They show karstic features and differ by volume, much less by area. At the occasion of an excursion to Lake Prespa in September 1994, an absence of oxygen in the hypolimnion between 17 m and the maximum lake depth, 48 m, was observed, for the first time since Stankovic' (1926) record. Moreover, a continuous decrease in lake level since the 1960s, the lowest transparency ever recorded from Megali Prespa, and high nutrient values, though only observed during a short part of the late stagnation period, are alarming signals, all suggesting eutrophication.     

The decline of the native fishes of lakes Victoria and Kyoga (East Africa) and the impact of introduced species,especially the Nile perch,Lates niloticus,and the Nile tilapia,Oreochromis niloticus

Synopsis There has been a decline, and in some cases an almost total disappearance, of many of the native fish species of lakes Victoria and Kyoga in East Africa since the development of the fisheries of these lakes was initiated at the beginning of this century. The Nile perch, Lates niloticus, a large, voracious predator which was introduced into these lakes about the middle of the century along with several tilapiine species, is thought to have caused the reduction in the stocks of several species. But overfishing and competition between different species also appear to have contributed to this decline. By the time the Nile perch had become well established, stocks of the native tilapiine species had already been reduced by overfishing. The Labeo victorianus fishery had also deteriorated following intensive gillnetting of gravid individuals on breeding migrations. L. niloticus is, however, capable of preying on the species which haven been overfished and could have prevented their stocks from recovering from overfishing. L. niloticus is also directly responsible for the decline in populations of haplochromine cichlids which were abundant in these lakes before the Nile perch became established. Even without predation by Nile perch, it has been shown that the haplochromine cichlids could not have withstood heavy commercial exploitation if a trawl fishery had been established throughout Lake Victoria. Their utilisation for human food has also posed some problems. The abundance of the native tilapiine species may also have been reduced through competition with introduced species which have similar ecological requirements. At present, the Nile perch and one of the introduced tilapiine species, Oreochromis niloticus, form the basis of the fisheries of lakes Victoria and Kyoga.Invited editorial     

Interactions of pelagic cnidarians and ctenophores with fish: a review

Medusae, siphonophores and ctenophores (here grouped as `pelagic coelenterates') interact with fish in several ways. Some interactions are detrimental to fish populations, such as predation by gelatinous species on pelagic eggs and larvae of fish, the potential competition for prey among pelagic coelenterates and fish larvae and zooplanktivorous fish species, and pelagic coelenterates serving as intermediate hosts for fish parasites. Other interactions are positive for fish, such as predation by fish on gelatinous species and commensal associations among fish and pelagic coelenterates. The interactions range from beneficial for the gelatinous species (food, parasite removal), to negative (predation on them). We review existing information and present new data on these topics. Although such interactions have been documented frequently, the significance to either fish or pelagic coelenterate populations is poorly understood. The effects of pelagic coelenterates on fish populations are of particular interest because of the great importance of fisheries to the global economy. As fishing pressures mount, it becomes increasingly important to understand how they may influence the balance between pelagic coelenterates and fish.     

Impact of the Nile perch on the fisheries of Lakes Victoria and Kyoga

The combined effects of lack of effective management, over-exploitation with destructive fishing gear and interspecific competition, particularly among tilapiines have had profound effects on the fish stocks of lakes Victoria and Kyoga. It has been proposed that these have been more important in the decline of the indigenous fisheries than predation or competition from Nile perch.     

Searching for responsible and sustainable recreational fisheries in the Anthropocene

Recreational fisheries that use rod and reel (i.e., angling) operate around the globe in diverse freshwater and marine habitats, targeting many different gamefish species and engaging at least 220 million participants. The motivations for fishing vary extensively; whether anglers engage in catch-and-release or are harvest-oriented, there is strong potential for recreational fisheries to be conducted in a manner that is both responsible and sustainable. There are many examples of recreational fisheries that are well-managed where anglers, the angling industry and managers engage in responsible behaviours that both contribute to long-term sustainability of fish populations and the sector. Yet, recreational fisheries do not operate in a vacuum; fish populations face threats and stressors including harvest from other sectors as well as environmental change, a defining characteristic of the Anthropocene. We argue that the future of recreational fisheries and indeed many wild fish populations and aquatic ecosystems depends on having responsible and sustainable (R&S) recreational fisheries whilst, where possible, addressing, or at least lobbying for increased awareness about the threats to recreational fisheries emanating from outside the sector (e.g., climate change). Here, we first consider how the concepts of R&S intersect in the recreational fishing sector in an increasingly complex socio-cultural context. Next, we explore the role of the angler, angling industry and decision-makers in achieving R&S fisheries. We extend this idea further by considering the consequences of a future without recreational fisheries (either because of failures related to R&S) and explore a pertinent case study situated in Uttarakahand, India. Unlike other fisheries sectors where the number of participants is relatively small, recreational angling participants are numerous and widespread, such that if their actions are responsible, they have the potential to be a key voice for conservation and serve as a major force for good in the Anthropocene. What remains to be seen is whether this will be achieved, or if failure will occur to the point that recreational fisheries face increasing pressure to cease, as a result of external environmental threats, the environmental effects of recreational fishing and emerging ethical concerns about the welfare of angled fish.     

Human-induced changes in the composition of fish communities in the African Great Lakes

Summary Large natural African lakes contain unique and diverse fish faunas which have evolved within each lake in a comparatively short period of time. members of the family Cichlidae are particularly diverse, although there is strong evidence to show that the haplochromines in Lake Victoria, and possibly Lake Malawi, are monophyletic. The unique faunas in Lakes Victoria and Kyoga have been subject to perturbations from the introduction of exotic fish, and the faunas in these and other lakes have been disturbed by fishing activities and other human endeavours.Factors governing the establishment of exotic species are not clearly understood. The exotic fish must be physiologically adapted to their new environment, able to compete successfully both for habitat and for food at each stage of their life history, able to avoid predation and must have a suitable reproductive potential. Although about 50 species of fish have been introduced into African inland waters, including reservoirs, only comparatively few, in particular Nile perch (Lates niloticus), various cichlids (especially tilapias) and clupeids (Limnothrissa miodon), have been successful in establishing themselves. Those that have become established have had obvious but unquantifiable impacts on the indigenous faunas.It is difficult to differentiate between the effects of fishing and of the presence of alien fish on the fish species composition of the lakes (Witte et al., 1992). Many of the lakes were overfished before introductions were made, with a resultant decline in some species, especially the larger ones, and the virtual disappearance of others. Some lake fish faunas, such as those of Lakes Kyoga and Victoria, which have been subjected to the perturbations described above, continue to change rapidly (Ogutu-Owayo, 1990b).There is a fundamental need to collect biological information on the fish communities of African lakes for effective management, resulting not only in the conservation of unique fish faunas but also the production of sustainable fish yields for the people relying on this source of protein. This information is required before any more introductions of exotic fish are made.     

Coastal Fisheries in the Eastern Baltic Sea (Gulf of Finland) and Its Basin from the 15 to the Early 20th Centuries

The paper describes and analyzes original data, extracted from historical documents and scientific surveys, related to Russian fisheries in the southeastern part of the Gulf of Finland and its inflowing rivers during the 15- early 20^th centuries. The data allow tracing key trends in fisheries development and in the abundance of major commercial species. In particular, results showed that, over time, the main fishing areas moved from the middle part of rivers downstream towards and onto the coastal sea. Changes in fishing patterns were closely interrelated with changes in the abundance of exploited fish. Anadromous species, such as Atlantic sturgeon, Atlantic salmon, brown trout, whitefish, vimba bream, smelt, lamprey, and catadromous eel were the most important commercial fish in the area because they were abundant, had high commercial value and were easily available for fishing in rivers. Due to intensive exploitation and other human-induced factors, populations of most of these species had declined notably by the early 20^th century and have now lost commercial significance. The last sturgeon was caught in 1996, and today only smelt and lamprey support small commercial fisheries. According to historical sources, catches of freshwater species such as roach, ide, pike, perch, ruffe and burbot regularly occurred, in some areas exceeding half of the total catch, but they were not as important as migrating fish and no clear trends in abundance are apparent. Of documented marine catch, Baltic herring appeared in the 16^th century, but did not become commercially significant until the 19^th century. From then until now herring have been the dominant catch.     

Impact of 21st century climate change on the Baltic Sea fish community and fisheries

The Baltic Sea is a large brackish semienclosed sea whose species-poor fish community supports important commercial and recreational fisheries. Both the fish species and the fisheries are strongly affected by climate variations. These climatic effects and the underlying mechanisms are briefly reviewed. We then use recent regional – scale climate – ocean modelling results to consider how climate change during this century will affect the fish community of the Baltic and fisheries management. Expected climate changes in northern Europe will likely affect both the temperature and salinity of the Baltic, causing it to become warmer and fresher. As an estuarine ecosystem with large horizontal and vertical salinity gradients, biodiversity will be particularly sensitive to changes in salinity which can be expected as a consequence of altered precipitation patterns. Marine-tolerant species will be disadvantaged and their distributions will partially contract from the Baltic Sea; habitats of freshwater species will likely expand. Although some new species can be expected to immigrate because of an expected increase in sea temperature, only a few of these species will be able to successfully colonize the Baltic because of its low salinity. Fishing fleets which presently target marine species (e.g. cod, herring, sprat, plaice, sole) in the Baltic will likely have to relocate to more marine areas or switch to other species which tolerate decreasing salinities. Fishery management thresholds that trigger reductions in fishing quotas or fishery closures to conserve local populations (e.g. cod, salmon) will have to be reassessed as the ecological basis on which existing thresholds have been established changes, and new thresholds will have to be developed for immigrant species. The Baltic situation illustrates some of the uncertainties and complexities associated with forecasting how fish populations, communities and industries dependent on an estuarine ecosystem might respond to future climate change.     

Ecological, environmental and socioeconomic aspects of the Lake Victoria's introduced Nile perch fishery in relation to the native fisheries and the species culture potential: lessons to learn

Inland fishery ecosystems in Africa are characterized by patterns of overexploitation, environmental degradation and exotic species introductions. Ecological complexity and diversity of aquatic habitats dictate that fishes in general are not evenly distributed in a water body. However, fisheries management regimes tend to ignore this basic principle, assume generalized conditions in a water body, and focus more on ‘desired’ objectives such as maximizing catch. The result is to disregard fish habitat boundaries and anthropogenic influences from the catchment that influence fish production. Overexploitation and environmental degradation disrupt sustainable socioeconomic benefits from the fisheries, create uncertainty among investors, but leave some managers calling for more information with the expectation that the fisheries will recover with time. Open access to the fisheries and full control of fishing effort remain challenges for managers. Exotic species introductions and fish farming can increase production, but such interventions require firm commitment to sound ecological principles and strict enforcement of recommended conservation and co‐management measures in capture fisheries. The general tendency to downplay fishing effort issues, other ecosystem values and functions or rely on temperate fisheries models until a new cycle of overexploitation emerges, characterizes many management patterns in inland fisheries. Aquaculture is not an option to challenges in capture fisheries management. Aquaculture should be developed to increase fish production but even this practice may have negative environmental impacts depending on practice and scale. Decades of information on Lake Victoria fisheries trends and aquaculture development did not stop the collapse of native fisheries. The successfully introduced Nile perch (Lates niloticus) has shown signs of overexploitation and aquaculture has again been considered as the option. By reviewing significant trends associated with Nile perch and its feasibility in aquaculture this paper uses Lake Victoria to illustrate ‘special interest management’ targeting selected species of fish rather than the fisheries.     

Species composition and seasonal cycles of phytoplankton with special reference to the nanoplankton of Lake Mikri Prespa

The phytoplankton of Lake Mikri Prespa was studied atmonthly or biweekly intervals during the period May1990–September 1992. Its species composition,consisting of a great number of cyanophytes and a verysmall number of chrysophytes and desmids, may reflectthe eutrophic character of the lake. Moreover, themean annual biomass values (15.0 and 3.2 g m^–3 inthe two years, respectively) and the maximum biomass(38.1, 6.4 and 9.6 g m^–3), classify Mikri Prespaas a eutrophic lake. A tendency towards adouble-peaked pattern of biomass distribution in timewith one peak in autumn, composed mainly ofcyanophytes, and another in spring made up of diatoms,was observed. This pattern contrasts with the standardpattern in eutrophic, stratified temperate lakes,which exhibit a third biomass maximum in summer.Cyanophytes were the most important group in terms ofbiomass and were dominated by the species Microcystis aeruginosa, Microcystis wesenbergii,Anabaena lemmermannii var. minor and Aphanocapsa elachista var. conferta. Diatomsconstituted the second most important group, with main representative the species Cyclotellaocellata. Cyanophytes, diatoms, chlorophytes anddinophytes revealed annual periodicity whereas theother algal groups did not show any seasonality atall.The nanoplankton constituted an important part ofalgal biomass (38.9 and 49.9% in the two years,respectively) and revealed annual periodicity withmaximum values in winter and spring, mainly composedof diatoms and cryptophytes. Low temperature,increased rainfall and high DIN concentrations seemedto be the main factors influencing the seasonality.Although the percentage contribution of nanoplanktondecreased with the increase in total biomass,justifying the classification of Lake Mikri Prespaamong the eutrophic lakes, the nanoplankton biomassdid not correlate significantly with totalphytoplankton biomass.